One way to keep tomato plants healthy, especially when grown in a confined space such as a greenhouse or where they may be planted close together, is to increase aeration.
This particularly applies to the bottom part of the plant where old leaves and soil may infect the upper, healthy part of the plant and cause disease.
De-leafing old, lower leaves improves air circulation and making sure that watering doesn't splash soil up onto the lowest leaves, also helps to avoid infection. Watering into a pot sunken into the soil will help to avoid this.
It's also a good idea to pull off the leaf branches by hand rather than cut them because knives and scissors can spread disease from one plant to another.